Zinc Coating Structure & Corrosion: A Comparative Study

Surface & Coatings Technology 200 (2006) 6594 – 6600
www.elsevier.com/locate/surfcoat
A comparative study of the structure and the corrosion behavior of zinc
coatings deposited with various methods
G. Vourlias a , N. Pistofidis a , D. Chaliampalias a , E. Pavlidou a , P. Patsalas b , G. Stergioudis a,⁎,
D. Tsipas c , E.K. Polychroniadis a
a
Aristotle University of Thessaloniki, Department of Physics, GR 54124 Thessaloniki, Greece
University of Ioannina, Department Materials Science and Engineering, GR 45110 Ioannina, Greece
Aristotle University of Thessaloniki, Department of Mechanical Engineering, GR 54124 Thessaloniki, Greece
b
c
Available online 13 December 2005
Abstract
The structure and the corrosion performance of Zn coatings formed by pack cementation and fluidized bed reactor at 250, 350, 380 and 400 °C
have been studied. From this investigation, it turned out that these coatings are composed of two layers referring to γ-Fe11Zn40 and δ-FeZn10
phases of the Fe–Zn phase diagram. Furthermore, in the pack coatings, inclusions were detected, which are composed of Fe and Zn at almost
equal concentrations. Concerning the corrosion, a mechanism of stress corrosion has been identified. The corrosion performance of the studied
coatings is similar to that of the hot-dip galvanized coatings, although the deposition method is different.
© 2005 Elsevier B.V. All rights reserved.
Keywords: Surfaces and interfaces; Coating materials; Metals and alloys; Scanning electron microscopy; X-ray diffraction
1. Introduction
Atmospheric corrosion, as far as it regards ferrous materials,
is one of the most important issues. The repair of the damages
that this phenomenon causes costs several billions of Euros
nowadays, without taking into account the environmental impact and the loss of natural resources [1,2]. Therefore, many
methods have been studied to overcome it ([1,2] and references
therein).
Zn metal has a number of characteristics that make it well
suited for use as a protective coating for ferrous substrates and
as a result Zn coatings are widely used for corrosion control [3–
6]. Their performance results from their ability to react with the
atmospheric compounds (O2, CO2 and H2O) which form over
the coating surface dense, adherent films, whose rate of corrosion is considerably below that of ferrous materials (barrier
protection). Additionally, Zn is anodic to iron and steel and
consequently offers cathodic protection.
A number of different methods of casting Zn coatings are
commercially available. The most widely used is hot-dip galva⁎ Corresponding author. Tel.: +30 2310 998 085; fax: +30 2310 998 003.
E-mail address: [email protected] (G. Stergioudis).
0257-8972/$ - see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.surfcoat.2005.11.039
nizing [3–6]. In this case, the ferrous substrate is immersed in a
bath of molten Zn and as a result it is covered by a layer of Zn
with an average thickness of a few tens of micrometers. The
effectiveness of this method is undisputable. However, its high
environmental impact imposes the investigation of alternative
coating techniques friendlier to the environment. A promising
technique that could be used instead of hot-dip galvanizing is
chemical vapor deposition (CVD) by pack cementation [7]. In
this case, a Zn diffusion coating is formed by heating the
substrate up to 400 °C covered with a mixture of powders
containing Zn. A similar method is CVD in fluidized bed reactor
(FBR) [8]. In this case, the substrate is immersed in a fluidized
bed of particles containing Zn and heated up to 400 °C.
Although CVD is widely used for the deposition of aluminum, chromium, silicon and other elements on different substrates in industrial-scale facilities [7–17], its application for
Zn deposition is relatively rare. As a matter of fact, only the
Table 1
Chemical composition (wt.%) of SAE 1010 specimens
Fe
C
Mn
S
P
Si
Al
Balance
0.11
0.55
0.012
0.016
0
0
G. Vourlias et al. / Surface & Coatings Technology 200 (2006) 6594–6600
Table 2
Pack cementation conditions
Zn
(wt.%)
NH4Cl
(wt.%)
Al2O3
(wt.%)
Temperature
(°C)
Heating duration
(min)
50
2
5
2
5
2
5
2
5
2
5
2
5
2
5
2
5
2
5
48
45
48
45
48
45
48
45
28
25
28
25
28
25
28
25
28
25
250
240
350
240
380
240
400
240
400
240
400
120
400
60
400
30
400
15
50
50
50
70
70
70
70
70
6595
time and powder composition). Furthermore, since hot-dip
galvanizing is mainly used for corrosion protection, induced
from atmospheric exposure, the anticorrosive performance of
the as-casted coatings is studied in a salt spray chamber,
where the atmospheric corrosion phenomena are highly
accelerated.
This investigation is of great importance because it offers
important data for the scaling up of these methods to industrial
applications. Furthermore, the summary of data concerning the
estimated service life of Zn diffusion coatings, as it is determined with accelerated corrosion tests, could assist the preliminary economic assessment of these methods.
2. Experimental procedure
formation of a two-step Al and Zn diffusion coating is
reported [18]. In this case, a Zn layer formed with sherardizing (mechanical plating [19]) is covered by an Al layer
formed by pack cementation. The corrosion resistance of
Zn–Al coatings formed with this method has been studied
elsewhere [20].
The present work deals with a systematic research on the
microstructure of Zn diffusion coatings formed with pack
cementation and FBR. The effect of different processing
factors is studied (temperature of the coating process, heating
Commercial, hot-rolled low carbon steel sheets SAE 1010
were used as substrates for the sample preparation. The chemical composition as analysed by mass spectrometry (SPECTROLAB M8 mass spectrometer) is listed in Table 1. The
samples were sized to 15 mm × 10 mm × 2.2 mm and then
polished progressively by hand finishing up to 600-grit SiC
paper.
Some of the as-prepared samples were coated by pack
cementation. For that purpose, they were placed in porcelain
crucibles filled by powder mixtures with the formulations
summarized in Table 2. The crucibles were covered with
porcelain lids, sealed with high-temperature resistant cement
and placed in a tubular argon-purged electric furnace, which
was preheated at the temperature of the coating formation.
The heating duration is presented in Table 2. After the
heating period, the crucibles were left in the furnace to
Fig. 1. SE micrographs of pack coatings formed at 250 °C (a), at 350 °C (b), at 400 °C with Zn concentration 50 wt.% (c), and at 400 °C with Zn concentration 70 wt.
% (d).
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G. Vourlias et al. / Surface & Coatings Technology 200 (2006) 6594–6600
Element concentration (mol%)
100
δ -FeZn10
γ -Fe11 Zn40
Zn
80
60
(1)
40
20
Fe-Zn Interface
0
0
2
4
Fe
6
8
10
12
14
Fig. 4. SE micrograph of the cross section of a coating formed in FBR at 400 °C
with 70 wt.% Zn.
16
Distance from the surface (μm)
Fig. 2. Compositional profile along the cross section of a pack coating formed at
400 °C with 50 wt.% Zn after a heating time of 240 min. The peak noted with
(1) refers to a Fe–Zn inclusion.
cool down to ambient temperature without interrupting the
Ar flow.
Other samples were coated by FBR using a typical apparatus
described previously [8]. The deposition temperature was 400
°C, the powder mixture was composed of 70 wt.% Zn, 5 wt.%
NH4Cl and 25 wt.% Al2O3, the heating duration was 1 h and
the fluidization gas was Ar.
In both processes, the coating is formed from the gas phase
in a minimum three reaction steps. The first one includes
NH4Cl decomposition to NH3 and HCl. The second refers to
the sum of composition reactions where Zn compounds with Cl
and NH3 are formed, while the third one refers to the sum of
decomposition reactions which lead to the deposition of Zn on
the ferrous substrate. Al2O3 does not take part in any of these
reactions. This mechanism is the same regardless of the deposition methods used in this work (pack cementation or fluidized
bed reactor).
1
1
INTENSITY (ARBITRARY UNITS)
The corrosion of the specimens took place in a Salt Spray
Chamber SC-450. The corrosive medium was 5 wt.% NaCl in
de-ionized water. The chamber temperature was 40 °C and the
relative humidity 100%. Specimens were retrieved from the
chamber after 24, 48, 96, 144, 192 and 240 h. In the chamber,
simultaneously with the pack coated samples, were placed hotdip galvanized samples on the same steel substrate for comparison reasons, which were also retrieved after 24, 48, 96, 144,
192 and 240 h.
For the examination of the microstructure and the corrosion
behavior, cross sections from each specimen have been cut
before and after the corrosion test, mounted in bakelite,
polished down to 5-μm alumina emulsion and etched in a 2%
Nital solution. The nature of the phases was determined using
X-ray diffraction (XRD), while the observation of the coatings
took place with scanning electron microscope (SEM) associated with an Energy-Dispersive X-ray Spectroscopy (EDS) analyser. The XRD experiments were done in a conventional
powder diffractometer (SEIFERT 3003 TT-CuKα radiation) in
Bragg–Brentano geometry; therefore, the XRD patterns originate from the whole coating volume and do not discriminate
the contributions from the surface and buried layers. No
2
γ phase
δ phase
1
2
1
1
1
1
1
1
1
2
1
1
30
1
1
1
1
2
40
50
2θ degrees
60
1
2
1
2
70
0
Fig. 3. XRD pattern of pack coatings formed at 400 °C after a heating time of 240 min. The peaks noted with (1) refer to γ-Fe11Zn40 phase and the peaks noted with
(2) refer to δ-FeZn10 phase.
G. Vourlias et al. / Surface & Coatings Technology 200 (2006) 6594–6600
6597
Fig. 5. SE micrographs of the cross section of corroded pack coatings [(a) after 24 h, (b) after 48 h, (c) after 240 h].
contribution from the stainless steel substrate has been recorded
in the XRD patterns, as determined by the penetration depth of
the X-ray beam following the analysis described in Ref. [21].
For the SEM study, the specimens were examined with a
20-kV JEOL 840A SEM equipped with an OXFORD ISIS
300 EDS analyzer and the necessary software in order to
Fig. 6. Chemical mapping of the interface of corroded pack coating after 144 h of exposure [(a) SE micrograph, (b) O distribution, (c) Zn distribution, (d) Cl
distribution].
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G. Vourlias et al. / Surface & Coatings Technology 200 (2006) 6594–6600
coating appears to be much worst than expected. Although it is
more compact and the uncoated areas on the substrate are much
smaller, many cracks remain.
At 400 °C coating integrity comparable to the integrity of
the hot-dip galvanized coatings (Fig. 1c) has been achieved.
The thickness of the coating is homogeneous (about 80 μm in
average). This thickness is likely to offer equivalent corrosion
protection to the hot-dip galvanised coatings because it is of the
same order of magnitude [3–6].
Fig. 2 shows a representative EDS line scan of these coatings, which reveals the distribution of Fe and Zn in the coating
cross section. From these data and micrographs similar to those
of Fig. 1c and d, it is evident that the coating has a two-layer
structure, with a surface layer, ∼3.5 μm thick, enriched in Zn.
The Zn concentration, as it is calculated from the diagram of
Fig. 2, implies that the buried (inner) layer refers to the γFe11Zn40 while the surface (outer) layer corresponds to the δFeZn10 phases of the Fe–Zn phase diagram, since the Zn
amounts to about 20–25 wt.% in the inner and 8–12 wt.% in
the outer layer [3]. The formation of these phases was probably
accomplished through Zn inward diffusion in the ferrous substrate. If outward diffusion was predominant, inclusions of
alumina would be observed in the coating cross section originating from the powders of the pack [7]. The structure of the
coating was verified by XRD (Fig. 3). The XRD pattern corresponds exclusively to γ-Fe11Zn40 (peaks denoted by 1) and δFeZn10 (peaks denoted by 2).
Several inclusions were also observed in the pack coatings
(the composition of such an inclusion is also recorded as the
abrupt peak in the EDS line scan of Fig. 2). Their EDS analysis
shows that they are composed by Fe and Zn at almost equal
concentrations. Consequently, they should refer to a Fe–Zn solid
solution, and as a result, no peaks were recorded at the XRD.
The above-mentioned phases compose also the pack coatings formed by different formulations and at different heating
times (Fig. 1d). The heating time variations seem to affect only
Fig. 7. Element distribution at the cross section of a corroded pack coating
formed at 400 °C with 50 wt.% Zn after a heating time of 240 min.
perform point microanalysis, linear microanalysis or chemical
mapping of the surface under examination.
3. Results and discussion
3.1. Characterization of the coatings prior to corrosion
Some typical micrographs of pack coatings cross sections
prior to corrosion are presented in Fig. 1. From these micrographs, it is obvious that the coating quality formed at 250 °C is
very poor (Fig. 1a). The coating thickness is inhomogeneous,
large areas of the ferrous substrate are exposed and many cracks
are observed. In any case, coatings of that quality are useless
for any kind of applications.
The coating quality is improved as the temperature increases
at 350 °C (Fig. 1b) and 380 °C. However, even in this case, the
INTENSITY (ARBITRARY UNITS)
1
γ-phase
3
3
31
11
30
1
3
3
3
3
2
1
40
1
1
50
2θ
3
1 1
3
1
2 3
60
3
70
degrees0
Fig. 8. XRD pattern of the surface of a corroded pack coating formed at 400 °C with 50 wt.% Zn after a heating time of 240 min and 8 days corrosion (1: ZnO, 2: iron
oxides, most probably Fe2O3 or FeO, 3: zinc hydrated chlorides).
G. Vourlias et al. / Surface & Coatings Technology 200 (2006) 6594–6600
6599
the coating thickness, which decreases as heating duration
decreases. The effect of the formulation is not quite obvious.
It seems that too low variations of the pack composition are
ineffective.
The coatings formed in FBR are characterized by the same
microstructure than that of the pack coatings (Fig. 4). Two
layers are also observed with compositions referring to the δFeZn10 for the surface layer and to the γ-Fe11Zn40 phase for the
buried layer. The coating thickness is of the same order of
magnitude. However, the FBR-produced coatings are more
uniform, since no inclusions of other phases are observed.
3.2. Characterization of the coatings after corrosion
The corrosion of the pack specimens was severe. As the
micrographs of Fig. 5 show, although after 24 h, no degradation
is observed, after 48 h, small cracks appear in the coating. The
size and the density of these cracks increases as the exposure
time increases, leading finally to the delamination of the coating (Fig. 5c). These cracks are also the preferred paths for the
penetration of chloride and oxygen ions in the coating as the
chemical mapping of Fig. 6 and the EDS line scan of Fig. 7
shows. The same phenomena were also observed for the coatings formed in FBR.
The presence of cracks implies that the initiation of corrosion could be ascribed to a mechanism of stress corrosion [1,2].
In the case of hot-dip galvanized coatings, the mismatch between the thermal expansion coefficients of the Fe–Zn phases
of the coating and the iron substrate leads to the development of
tensile residual stresses upon cooling, which in the case of
galvanizing usually takes place by exposing the coated object
in the air. These stresses are relieved through the formation of
cracks inside the Zn coating [22]. The crack formation is
already accomplished when the coating is exposed in the atmosphere. The coating thus contains a pre-existing crack network,
which, as far as it concerns hot-dip galvanized coatings, is
accumulated mainly in the δ-FeZn10 phase [22].
Since the outer phase of the Zn diffusion coatings, which are
studied in the present work, is the δ-FeZn10, it is likely that the
surface of the coating is characterized by a number of crack
tips, formed with the same mechanism as it was previously
mentioned for the hot-dip galvanized coatings. The cracks offer
paths to chloride ions which are dissolved in the corrosive
medium. The Cl− ions and oxygen which penetrate into the
coating react with Zn [1,2] resulting in the Zn dissolution
through the formation of soluble materials as Zn hydrated
chlorides (Fig. 8). Furthermore, ZnO is formed (Fig. 8) whose
specific volume is much higher than the specific volume of Zn.
Consequently, the widening of the cracks is enhanced. These
phenomena finally lead to the decomposition of the coating.
By contrast, the corrosion of hot-dip galvanized coatings is
almost uniform (Fig. 9). This could be explained by the fact that
in this case, the δ-FeZn10 phase is not exposed to the corrosive
medium. The outer phase of the hot-dip galvanized coatings is
the eta (η) phase which is composed by almost pure Zn. The eta
phase is characterized by low hardness unlikely to the Fe–Zn
phases that form the Zn pack coatings [3]. Consequently, the
Fig. 9. SE micrograph of the cross section of corroded hot-dip galvanized
coatings after 144 h of exposure.
crack propagation in this phase is rare, and as a result, no stress
corrosion occurs. However, although the corrosion mechanism
differs, the attack of the galvanized coating seems to be also
very severe. After 8 days of exposure, a large volume of
corrosion products is gathered on their surface, while Zn chlorides and oxides are formed in the coating, as EDS showed.
Generally, there is a steady degradation of the coating as the
exposure time increases. Hence, it could be deduced that the
corrosion performance of the above-mentioned types of Zn
coatings is similar.
4. Conclusions
The following conclusions have been drawn in this work.
1. The Zn coatings formed by pack cementation and FBR at
temperature 400 °C are composed by two layers referring to
the γ-Fe11Zn40 and the δ-FeZn10 phases of the Fe-Zn phase
diagram.
2. In the pack coatings, inclusions were detected which are
composed by Fe and Zn at almost equal concentrations. As
far as it concerns these conclusions, further research is
needed, in order to reveal their contribution to the mechanical and the anti-corrosive behavior of the coating.
3. The corrosion of the diffusion coatings could be ascribed to
a mechanism of stress corrosion. However, in order to draw
a conclusion beyond any doubt, a microscopic re-examination of the non-corroded pack and FBR coatings is necessary
to verify the formation of cracks during their deposition.
4. The corrosion performance of the Zn coatings studied in this
work is almost similar to the corrosion performance of the
hot-dip galvanized coatings.
Acknowledgments
This project was partially financed by the Greek Ministry of
National Education through the program Pythagoras II (project
no. 80832).
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